Warning for Geo-engineers: SO2 may surprise you

November 30, 2010

I’ve blogged before about trying to avoid geo-engineering as a solution to climate change. We just don’t know enough about the complexities of climate to try relatively simple fixes, like adding sulfur dioxide to the atmosphere. The following news release from the European Space Agency’s Venus Express program ought to give us pause.

A mysterious high-altitude layer of sulphur dioxide discovered by
ESA’s Venus Express has been explained. As well as telling us more
about Venus, it could be a warning against injecting our atmosphere
with sulphur droplets to mitigate climate change.

Venus is blanketed in sulphuric acid clouds that block our view of the
surface. The clouds form at altitudes of 50-70 km (30-45 miles) when
sulphur dioxide from volcanoes combines with water vapor to make
sulphuric acid droplets. Any remaining sulphur dioxide should be
destroyed rapidly by the intense solar radiation above 70 km.

So the detection of a sulphur dioxide layer at 90-110 km by ESA’s
Venus Express orbiter in 2008 posed a complete mystery. Where did that
sulphur dioxide come from?

Now, computer simulations by Xi Zhang, California Institute of
Technology, USA, and colleagues from America, France and Taiwan show
that some sulphuric acid droplets may evaporate at high altitude,
freeing gaseous sulphuric acid that is then broken apart by sunlight,
releasing sulphur dioxide gas.

“We had not expected the high-altitude sulphur layer, but now we can
explain our measurements,” says Hakan Svedhem, ESA’s Venus Express
Project Scientist.

“However, the new findings also mean that the atmospheric sulphur
cycle is more complicated than we thought.”

As well as adding to our knowledge of Venus, this new understanding
may be warning us that proposed ways of mitigating climate change on
Earth may not be as effective as originally thought.

The proposal stems from observations of powerful volcanic eruptions,
in particular the 1991 eruption of Mount Pinatubo in the Philippines
that shot sulphur dioxide up into Earth’s atmosphere. Reaching 20 km
in altitude, the gas formed small droplets of concentrated sulphuric
acid, like those found in Venus’ clouds, which then spread around
Earth. The droplets created a haze layer that reflected some of the
Sun’s rays back into space, cooling the whole planet by about 0.5
degree C.

However, the new work on the evaporation of sulphuric acid on Venus
suggests that such attempts at cooling our planet may not be as
successful as first thought, because we do not know how quickly the
initially protective haze will be converted back into gaseous
sulphuric acid: this is transparent and so allows all the Sun’s rays
through.

“We must study in great detail the potential consequences of such an
artificial sulphur layer in the atmosphere of Earth,” says Jean-Loup
Bertaux, Universite de Versailles-Saint-Quentin, France, Principal
Investigator of the SPICAV sensor on Venus Express. “Venus has an
enormous layer of such droplets, so anything that we learn about those
clouds is likely to be relevant to any geo-engineering of our own
planet.”

In effect, nature is doing the experiment for us, and Venus Express
allows us to learn the lessons before experimenting with our own
world.

# # #

“Photolysis of sulphuric acid as the source of sulphur oxides in the
mesosphere of Venus,” by Xi Zhang, Mao-Chang Liang, Franck Montmessin,
Jean-Loup Bertaux, Christopher Parkinson and Yuk L. Yung, is published
in Nature Geoscience today.

Venus Express’s instrument SPICAV stands for Spectroscopy for
Investigation of Characteristics of the Atmosphere of Venus.